Regulations to limit the amount of VOC content of industrial coatings have encouraged research and development to explore new technologies directed at reducing solvent emissions from industrial coatings operations such as automotive, appliance, general metal, furniture, and the like. One technology, the replacement of organic solvents with water, is of particular interest for the obvious reasons of availability, cost, and environmental acceptability.
Cross-linkable waterborne enamels have been prepared from each of the three primary industrial film-forming polymer types: polyesters, acrylics and alkyds. Each of these three polymer types can be made water dissipatable by neutralizing residual carboxylic acid groups (eg. formation of an amine salt) or incorporating a surfactant combination selected to make the resin/water mixture emulsifiable. The amine (viz. ammonia) volatilizes from the curing film which increases the VOC.
Amine salts of acrylic polymers have a stable carbon backbone, but pendant ester groups (methyl, butyl, .beta.-hydroxyethyl) are susceptible to saponification at high pH. The hydrolysis of pendant ester groups can lead to sediment formation and enamel instability.
Amine neutralized polyesters are also saponifiable; however, main chain bond cleavage results in loss of performance versus storage time. Scientists have attempted to retard or eliminate this effect through deliberate raw material selection to effect more stable ester linkages. It remains that, at best, amine neutralizable polyesters represent a short-term solution to provide the acknowledged benefits of waterborne coatings in the marketplace.
Thirdly, alkyd resins, with fewer ester linkages per unit weight due to the high weight contribution (45 to 85%) from C-18 fatty acid groups, are commercially available for use in manufacturing amine neutralized water-dissipatable paints. However, due to their oil-like or hydrophobic properties, alkyds require organic cosolvents for preparing satisfactory dispersions. The amount of solvent required is not insignificant with 1 pound or more of solvent per 3 pounds of alkyd resin being typical. Such high solvent demand makes it difficult to formulate compliant coatings with VOC target values less than 2.6 pounds organic volatiles per gallon of paint.
Some polymer types are known to be formed into aqueous, ion-containing polymer dispersions suitable for preparing storage stable paint. U.S. Pat. No. 4,340,519 discloses the composition of certain crystalline and non-crystalline polyesters copolymerized with a metal sulfonate group-containing aromatic acid and up to 10 mole percent (based on the total polyol content) of a polyvalent polyol selected from trimethylolpropane, trimethylolethane, glycerine, and pentaerythritol. U.S. Pat. No. 4,525,524 discloses liquid systems of polyesters containing certain metal sulfonates and, optionally, up to 3 percent of a branching agent based upon the total diol component.
Many patents disclose methods to obtain water-reducible polyesters and alkyds by neutralizing residual or unreacted carboxylic acid groups on the polymer with ammonia or various organic amines. U.S. Pat. No. 3,666,698 utilizes this method as well as phenylindandicarboxylic acid to modify coating performance. U.S. Pat. No. 3,699,066 shows the benefits of certain hydroxy-functional amines for neutralization. U.S. Pat. No. 3,549,577 utilizes the amino resin crosslinker as the neutralizing agent then adjusts the pH to prepare an alkaline water-reduced system. In these patents as well as U.S. Pat. Nos. 3,494,882, 3,434,987, BR. 1,117,126, and U.S. Pat. No. 3,345,313 carboxylic acid functionality is completely neutralized with excess base yielding an alkaline paint vehicle.
Although there are acceptable low VOC content polyesters and acrylics, the storage stability and resulting coating stability of polyesters and acrylics is significantly lower than alkyd resins. However, as stated above alkyds have high VOC content. It would, therefore, be very desirable to be able to produce a low VOC content alkyd resin that retains good storage stability and coating stability.